With the advance of electronics technology in recent years, miniaturization, lighter weight, and lower power dissipation of electronic equipment have become possible along with sophistication of functions. As a result, a variety of portable consumer electronics products have been developed and commercialized and the market size has been rapidly expanding as represented by camcorders, notebook-type personal computers, portable telephones, etc.
Further miniaturization and lighter weight as well as longer operating time of these equipment are in constant demand. In association with such demand, as built-in power source for these equipment, lithium secondary batteries as represented by long-life and high-energy density lithium-ion secondary battery have been actively developed and are in wide use. Under this trend, thin prismatic batteries are drawing attention as they are especially suitable for thinning of equipment and provide higher space efficiency.
The lithium-ion secondary battery is a battery system which employs a lithiated transition metal complex oxide as the positive active material, a graphite or carbon as the negative active material, and a non-aqueous electrolyte such as an organic electrolyte which is a solution of a lithium salt being an electrolyte in an organic solvent or a solid electrolyte being a conductor of lithium ions. When this battery is charged, lithium ions are extracted from the lithiated complex oxide of the positive electrode and dissolve into the electrolyte and at the same time lithium ions of the same electrochemical equivalent are inserted from the electrolyte into the carbon of the negative electrode. When discharged, conversely from the case of charging, lithium ions are inserted into the positive electrode to become a lithiated complex oxide and lithium ions are extracted from the negative electrode, and this process is repeated.
As the potential of the carbon negative electrode is close to the electrode potential of metallic lithium, a lithiated complex oxide of at least one transition metal element selected from the group consisting of cobalt, nickel, and manganese which gives a high electrode potential is generally used as the positive active material.
Lithium-ion secondary batteries have an advantage of having an outstandly high energy density per unit weight among currently commercialized battery systems, to say nothing of the energy density per unit volume. However, as these batteries use in most cases organic electrolyte, in the event of external short-circuit, overcharge, or overdischarge accompanying reverse charge, the battery temperature rises with the flow of current and the solvent in the organic electrolyte evaporates or decomposes, thereby causing a rapid and abnormal increase in the internal cell pressure leading to leakage of electrolyte.
Consequently, in order to prevent rapid and abnormal increase in the internal cell pressure, overcharge and overdischarge protection circuit is usually incorporated in a battery pack consisting of plural cells and at the same time an explosion-proof mechanism of which a safety vent is triggered in the event of an increase in the internal battery pressure to release the high-pressure gas inside the battery to the air, and a PTC element to prevent an excessive current from continuously flowing is provided in each cell. Under this circumstance, a mechanism has been proposed as disclosed, for example, in Japanese Laid-Open Patent No. Hei 2-112,151 in which a lead ribbon of an electrode plate is pre-provided to an explosion-proof vent which will be deformed by an increase of the internal pressure, and when the internal pressure reaches a predetermined value, the explosion-proof vent is deformed thus either breaking the lead ribbon or detaching it from the explosion-proof vent thereby cutting off the current. As this mechanism cuts off the current by making use of the increase of the internal cell pressure due to external short-circuit, overcharge, or overdischarge accompanying reverse charge, it is superior in principle as a means to prevent accident of explosion. It is especially effective in relatively large cylindrical or prismatic batteries. However, in the case of small-size batteries such as thin prismatic or cylindrical-type batteries with which the area of the seal plate is small this mechanism had a weak point of not being applicable because of both reliability and productivity as there is dispersion in the operating pressure.
The present invention provides a non-aqueous electrolyte battery provided an explosion-proof safety vent with a superior productivity and a high-reliability by adopting in small-size batteries having a small area of seal plate, especially lithium-ion secondary batteries which are thin and square or are oval in cross section, a seal plate having a relatively simple construction yet with a sure venting action at no sacrifice of the discharge capacity.